1. Field of the Invention
This invention relates, generally, to remediation of air pollution. More specifically, it relates to a photocatalytic remediation of indoor air pollution.
2. Brief Description of the Prior Art
Indoor air quality has become a great concern due to the increased time spent indoors.
Most people spend more than 90% of their time in indoor environments such as a home, office, car, shopping center, etc. [1, 2]. Studies show that the level of pollutants indoors is much higher than that of outdoor environments [3, 4]. Photocatalysis is becoming an extremely important method of disinfecting and cleaning indoor air. Photocatalysis is a promising technique for the remediation of air pollution because it is able to oxidize low concentrations of organic contaminants into benign products [5]. Photocatalysis utilizes semiconductors like TiO2, ZnO, WO3 or Fe2O3 to carry out a photo-induced oxidation process to breakdown volatile organic compounds (VOCs) and inactivate bacteria and viruses [6].
In order for photocatalysis to be effective, it is necessary for the contaminants to come in close contact with the photocatalyst on which a light of appropriate wavelength is incident. In other words, the effectiveness of this process is dependent on the probability of the contaminant particles being close to the catalytic surface. As the air flows past a catalytic surface, a boundary layer forms along the surface, which does not allow mixing of the air resulting in extremely low probability of contaminant particles coming in contact with the catalytic surface.
There are various photocatalytic reactors that have already been reported in the prior art, such as plate reactors and honeycomb reactors [7-16]. Most of the air reactors utilize a surface-coated catalyst configuration with the airflow being parallel to the catalyst surface. When the air flows parallel to a smooth catalytic surface, a laminar sublayer is formed over the surface that impedes the mass transfer of reactants to the catalyst and the reaction products to the main flow, thus adversely affecting the photocatalytic reaction rate. Moreover, indoor air pollutant levels are typically on the order of parts per million (ppm) [17, 18], which requires more mass transfer for effective heterogeneous photocatalysis. Although mass transfer plays a significant role in photocatalysis, it has not received much attention in the photocatalytic field. There are only a few research groups that have considered the effect of mass transfer in their photocatalytic study.
Mo et al. [19] developed a mass transfer units (NTUm) method in which they considered three key factors, including reactor area (A*), mass transfer (Stm), and reaction effectiveness (h), to study the performance of a photocatalytic reactor for removing VOCs. Passalia et al. [20] studied the photocatalytic degradation of formaldehyde and presented a non-linear expression based on mass balance and rate expression to estimate the kinetic parameters. Bimie et al. [21] investigated the influence of species mass transfer on the overall reaction rate of a flat plate photoreactor and developed a kinetic model incorporating the mass transfer theory.
Although some researchers have started to pay attention to the mass transfer in the kinetic models study, very few have studied how to increase the mass transfer in a photocatalytic reactor. The conventional method of increasing the mass transfer on the catalyst surface is by increasing the airflow rate in the reactor [22-24]. However, increasing the airflow rate reduces the residence time of the pollutants and leads to incomplete contaminant destruction and more intermediates. An ideal reactor design would be one that enhances the mass transfer rate of the contaminants to the catalyst surface and also increases the residence time.
Chen and Meng [25] developed a convective mass transfer field synergy equation with a specific boundary condition for photocatalytic oxidation reactors. By using the field synergy equation, the optimal velocity pattern in the reactor channel could be obtained. Based on the optimal flow pattern, they introduced discrete inclined ribs on the surface of the reactor channel to produce vortex flow pattern. Their experimental study showed that the contaminant removal effectiveness is increased compared to a smooth plate reactor. However, Chen and Meng failed to optimize the size, shape, or arrangement of the ribs to match the flow pattern in the reactor channel.
Vohra [26] described a commercially available reactor, in which the catalyst is coated on a non-uniform rough surface as the basis for his study of the effect of roughness on the turbulence of airflow on the surface. Vohra further stated that the catalytic coating on a rough surface was highly effective in the destruction of contaminants when compared to a similar reactor configuration without surface roughness elements. However, the study failed to determine the optimum surface roughness shape and geometry to maximize the effectiveness of photocatalytic air cleaning. Based on Vohra's study, it is assumed that roughness elements on the catalyst surface will increase the mass transport of the contaminants to the catalyst surface and thus lead to an increase in the effectiveness of photo catalysis.
The impact of surface roughness on the characteristics of fluid flow has been widely studied. It has been accepted that surface roughness elements have a great effect on the turbulent flow structure [27-30]. An early study, performed by Perry et al. [27], analyzed the effect of surface rib roughness on turbulent flow, comparing different pitch ratios of square roughness elements. The researchers proposed two primary types of square roughness elements that were classified as either d-type or k-type based on the pitch ratio (p/e), where ‘p’ is the pitch and ‘e’ is the height of roughness (See FIG. 1). The d-type roughness, in which the pitch ratio (p/e) is less than four, was characterized by a fully separated flow over the inter-rib vortex and thus did not affect the main flow. On the other hand, the k-type roughness, in which the pitch ratio (p/e) is greater than four, was characterized by a separated flow over the initial rib that became partially reattached before encountering the upstream face of the next rib and led to vortices and mixing eddies. More recently, emphasis has been given to numerical investigations of turbulent flow over rough surfaces to provide a better understanding of the turbulence characteristics. A computational study by Cui et al. [30] further explored the effect of rib spacing for turbulent channel flow exhibiting either d-type or k-type roughness. In d-type roughness, the separation eddies were confined to the gaps between the ribs. For k-type roughness, flow separation and reattachment occurs between two adjoining ribs. Subsequently, much larger and more frequent eddies are thrown into the outer flow, resulting in a strong interaction between the roughness layer and the outer flow. The experimental studies by Wang et al. [31] supported the results of Cui et al. [30].
A number of reports have shown that the roughness elements lead to an enhanced mass/heat transport [32-35]. The major effect of roughness elements is enhancing turbulent mixing [30]. An experimental study, conducted by Simonich and Bradshaw [34], reported a 5% increase in the heat transfer for every 1% increase in the turbulence intensity. Sanitjai and Goldstein [36] reported that the mass transfer (Sherwood number) increases about 60% as the free stream turbulence intensity increases by 23% in their experimental study of the effect of free stream turbulence on the local mass transfer from a circular cylinder. Moravejin et al. [37] also reported that mass transfer increased greatly with increased turbulence in their experimental investigation. Although the effect of surface roughness elements in enhancing heat/mass transfer is known, its effect on photocatalytic reactors has not been previously investigated.
Accordingly, what is needed is an improved photocatalytic reactor utilizing artificial roughness elements on the catalytic surface to enhance turbulence intensity, thus resulting in increased mass transfer and improved rate of photocatalysis. However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome.
All referenced publications are incorporated herein by reference in their entirety. Furthermore, where a definition or use of a term in a reference, which is incorporated by reference herein, is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
While certain aspects of conventional technologies have been discussed to facilitate disclosure of the invention, Applicants in no way disclaim these technical aspects, and it is contemplated that the claimed invention may encompass one or more of the conventional technical aspects discussed herein.
The present invention may address one or more of the problems and deficiencies of the prior art discussed above. However, it is contemplated that the invention may prove useful in addressing other problems and deficiencies in a number of technical areas. Therefore, the claimed invention should not necessarily be construed as limited to addressing any of the particular problems or deficiencies discussed herein.
In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or otherwise constitutes prior art under the applicable statutory provisions; or is known to be relevant to an attempt to solve any problem with which this specification is concerned.